Dumble 124 is this normal?

[neskor]

My good friend just bring his Dumble clone 124.
It was always quiter than mine 124 with the same input level.
Today I tried to measure AC signal levels on mine and his.
In a preamp I don't see the difference in signal levels, but signal after the phase inverter is different.
AC signal in front of the PI 0,02uF coupling cup is 500mV on both amps, after PI signal on 0.1uF coupling cup on mine is 10V AC on his 5,5V.
Tried different PI tube but the levels are the same, no change.
Then i tried to remove output tubes from his amp and I get 12v after PI same as on mine.
Aby idea where is the problem or this is normal behaviour?

[neskor]

I can see now that I posted in wrong sub forum, please move to Dumble Discussion.
Thanks!

[pompeiisneaks]

Moved.

I'd try measuring the components around that section of the amp. Something post PI may be shunting signal to ground. Maybe an incorrect component, etc. Looking at the schematic, all I can think of would be the 220k bias balance resistors off to the bias, or the B+3 node that provides the anodes power... Have you tried different Power Tubes? Maybe one is not doing great?

Either way confirm all the components post PI output to see if any are misbehaving... maybe one or both of the output filter caps is acting more like a resistor under load? but when tubes are pulled no current runs and therefore there's no 'drop'

~Phil

[sluckey]

1. Swap output tubes between amps. Does your amp now have the lower signal voltage?

2. Compare the bias settings for both amps.

3. Disconnect one end of the coupling caps from the output tube side. Check for dc voltage on the dangling end of those caps. Presence of voltage indicates a leaky cap which would make the output tube run hotter, maybe enough that the grid draws current. This will put a heavier load on the PI causing the signal voltage to drop.

[alkuz1961]

I think we should check the feedback circuits

[erwin_ve]

I think we should check the feedback circuits

That would explain the lower output voltage for the PI.

[bepone]

good point on different feedback! logical

[neskor]

you guys are correct!
different feedback resistor and output transformer tap
thanks!

[pompeiisneaks]

Ahh then if you want to keep it on that tap you need to adjust the resistor OR put it on the same tap

Sorry I didn't see that when I was looking over the schematic with my first post... DUH

~Phil